A metal shell of a spark plug is assembled to an insulator holding a center electrode, and an external thread is formed on an axial portion, of the metal shell, to which a flange portion is provided. The spark plug is attached to an engine by the external thread of the metal shell being screwed into a screw hole of the engine. The flange portion of the metal shell regulates the amount by which the external thread is screwed into the engine. The spark plug attached to the engine generates flame kernel in a spark gap between the center electrode and a ground electrode which is joined to the metal shell. In order to cause flame kernel to grow, the spark plug is preferably attached to the engine such that the spark gap is not hidden behind the ground electrode relative to an air flow generated in a combustion chamber in a compression step that is a pre-ignition step.
Incidentally, as the metal shell of the spark plug is screwed into the engine, the metal shell advances in the axial direction while rotating along a screw helix about the axis, until being regulated by the flange portion. The position of the ground electrode in the circumferential direction of the metal shell is determined at a position where the axial movement of the external thread is regulated by the flange portion. Therefore, the position of the ground electrode in the circumferential direction of the metal shell is dependent on the distance in the circumferential direction between the ground electrode and the cutting-start position of the external thread, and on the distance in the axial direction from the flange portion to the ridge of the external thread.
Japanese Patent Application Laid-Open (kokai) No. 2002-143969 discloses a technique for forming, through rolling, an external thread on an axial portion of a workpiece to which a ground electrode is joined. In this technique, in a state where a cutting-start position, in the circumferential direction, of the external thread is set, the distance in the axial direction between a flange portion and the cutting-start position of the external thread is set by use of a jig or an optical sensor.
However, in the technique disclosed in Japanese Patent Application Laid-Open (kokai) No. 2002-143969, it is required to reduce variation in the distance in the axial direction from the flange portion to the thread, in order to improve accuracy for the position of the ground electrode to be disposed in a combustion chamber.
The present invention has been made in order to meet the aforementioned need. An advantage of the present invention is a machining apparatus, a component producing method, and a spark plug producing method which enable reduction in variation in the distance in the axial direction from a flange portion and the ridge of a thread.